1. Field of the Invention
The invention relates to novel sintered refractory blocks, especially for the construction of aluminum electrolysis cells, to a method of manufacturing them, and to a cell comprising said blocks.
2. Description of the Related Art
As can be seen in FIG. 1, aluminum metal 2 may be produced on an industrial scale by electrolyzing alumina in solution in a bath 10 based on molten cryolite. The electrolyte bath 10 is conventionally contained in an electrolysis cell 12. The cell 12 comprises a side wall 14 and a bottom 16. The bottom 16 is composed of refractory bottom blocks 17 and cathode blocks 24 and insulating blocks in the lower portion. The side wall 14 is formed from refractory side blocks 18 surrounded by a metal casing 20.
The dimensions of a refractory side block 18 can vary. They are conventionally more than 30×100×100 mm [millimeter] and may attain 120×300×300 mm.
The composition of the blocks 18 may be based on carbon (graphite and/or anthracite). Typically, the mortar for the blocks 18 is a refractory cement 21 disposed between them and against the metal envelope 20. The cell 12 comprises at least one anode 22 and at least one cathode 24. The anodes 22 and cathodes 24 are disposed so as to be in contact with the molten metal bath, the cathode 24 conventionally being disposed close to the bottom 16.
When the electrodes 22 and 24 are placed under voltage, an electrolysis reaction occurs in the bath 10, resulting in the formation of a bath of aluminum in the cell, which bath is deposited on the cathode.
Passing high electric current through the bath 10 also causes heat to be released under the Joule effect. Evacuating that heat through the wall 14 of the cell 12 causes a layer 26 of solidified cryolite to be deposited on the inner surface 27 of the blocks 18. That layer is termed a “self-lining” layer.
The blocks 18 must protect the metal envelope 20 and allow sufficient heat to be evacuated to ensure temperature stabilization of the molten bath 10. In particular, it is vital to avoid reaching temperatures beyond which the self-lining layer 26 of solidified cryolite liquefies again and could contribute to very rapid corrosion of the sides of the cell. Further, the blocks 18 are often exposed to corrosive environments (very hot liquid metal, molten cryolite in the lower portion, corrosive gas in the upper portion), and they are subjected to high temperatures and large thermal and mechanical stresses.
To meet those challenges, blocks are known that are based on silicon carbide granulates which have generally satisfactory resistance to attack. Conventionally, silicon carbide granulates are sintered at a temperature in the range 1600° C. to about 2000° C. Sintering fine grained silicon carbide granulates at very high temperatures (2150° C.) is also known, enabling boron and carbon to be added. However, silicon carbide is very difficult to sinter and/or its cost is prohibitive. Further, the format of the sintered silicon carbide blocks is limited, especially due to a great deal of shrinkage on firing.
Blocks based on dense sintered silicon carbide granulates are also known, with less than 1% of B4C and C, for example Hexolloy SiC®. However, they are currently extremely expensive.
Finally, blocks based on silicon carbide (SiC) are known, bound by a matrix of silicon nitride (Si3N4). The materials for such blocks were developed at the end of the 1970s and are described, for example, in U.S. Pat. No. 2,752,258. They improve the compromise between oxidation resistance, mechanical strength (erosion), and thermal conductivity compared with carbon blocks. The improvement in abrasion resistance is particularly advantageous at the bottom of the cell where the bath, which moves under the effect of magnetic fields, may cause a great deal of abrasion.
Said blocks are obtained by reactive sintering of a mixture of silicon carbide and silicon, with nitrogen deriving from firing in a nitrogen atmosphere.
To gain useful volume and facilitate heat evacuation, research has been concentrated on reducing the thickness of such blocks. However, the thickness cannot be reduced without affecting the service life of the cells. Thus, it must be accompanied by an improvement in the oxidation resistance and resistance to attack by the cryolite bath. That need is greater if the stresses on the refractory blocks are higher. In particular, electrolysis cells are now used with a current of more than 200,000 amps and from which, as a result, a great deal of heat must be evacuated, large quantities of oxidizing gas are generated, and the self-lining layer may become unstable.
Thus, there is a need for a novel refractory block based on silicon carbide (SiC) with a nitride binder (Si3N4) that can effectively and durably resist the thermal and/or chemical stresses that may be produced in an aluminum electrolysis cell, in particular in the side wall thereof.